The manual suggests 87 and I've had the wife always follow that rule. When we were in AZ 87 was low grade, but up here in CO 85 is low grade, I'm assuming due to elevation. We live at 7300 ft and she commutes down to about 5500...can we get away with 85 at this elevation with no performance degradation, or should we stick with 87?

At higher elevations it will run fine, just the same as 87 at lower elevation. The air is not as dense up there and less air in the cylinder means lower cylinder pressures and less chance of engine knock. The lower air density is effectively the same as lowering the compression ratio of the engine allowing lower octane fuel. With that said you will still see the best performance from higher octane fuel.

I have a friend who is a Petrolium Engineer. He said the rule is you can run 1 octane point lower for each 1000 feet of altitude. That's whey even at 3300' I can't find higher than 91 octane and the majority of station's here are 90 octane for premium.

The manual suggests 87 and I've had the wife always follow that rule. When we were in AZ 87 was low grade, but up here in CO 85 is low grade, I'm assuming due to elevation. We live at 7300 ft and she commutes down to about 5500...can we get away with 85 at this elevation with no performance degradation, or should we stick with 87?

If you are running the stock ignition calibration, you will be fine with 85 and will see no performance improvement with 87 or 91. Should you decide to advance the timing, the higher octane will allow more advance without knock.

If you are running the stock ignition calibration, you will be fine with 85 and will see no performance improvement with 87 or 91. Should you decide to advance the timing, the higher octane will allow more advance without knock.

When bluegoat06 has posted in the past on the stock tune:
93 octane gasoline = no knock retard
91 octane gasoline = up to 5° of timing pulled
87 octane gasoline = up to 15° of timing pulled

I run Costco 91 octane gasoline with no ethanol.

I doubt that the above is true, unless it is pulled from context and is referring to a specific situation of short duration, or if he was talking about an LS3. I could be wrong, since I have not data logged an L76. (I have an LS3). I have datalogged 4 different GM cars up here in Denver with 85 octane and stock tune they did not log KR. (well, one did pull a little timing under load, but it was very short duration and only about 5 degrees.) I am also doubtful that GM would advance the timing on the stock tune such that it then pulls 15 degrees on the recommended fuel, that makes no sense to me (once again, unless it's an LS3).

I run Shell 91 100% of the time in my G8, I don't know if it's any better than anything else, it is "Top Tier" at least.

Also to clarify on the fact that this cars can run 87 octane. G8GT's were not tuned to run 87 from the factory. the 6.0L and 6.2L engines found in the G8 and GXP will run the best with 93 octane and the factory tune.

To give a brief explanation, there are 2 main tables that control engine timing in the GM calibration. A Hi-octane timing table and a Lo-octane timing table. as you guess, the Hi-octane have more timing compared to the Lo-octane table. The Hi-octane table is used as the main timing source during all engine operation conditions at part throttle and WOT. the lo-octane table is your car cushion for bad fuel and any other parameters that can induce engine knock. As the engine is knocking (most of the times you won't even hear the knock, GM is fairly good about this), knock sensors installed in your engine would be able to pick the knock sound, and command the PCM to move away from the Hi-octane table towards the Lo-octane table. it does this quickly enough for you to know the engine was knocking, and depending on the severity of the knock, more and more values from the Lo-octane table are used.

The Hi octane table was set from the factory with such aggressive values that the engine could not operate without the Lo-octane table and 87 octane. This is a fact, and having access to any PCM scanner that can show knock retard would easily prove it. and in case you're wondering, A GXP can run 87 octane in the same way a GT can run 87 octane. GM design engines and create tunes so this cars can operate in a a broad variety of conditions, a bone stock G8 would run in mohave desert conditions in the summer, or during winter conditions in Alaska. Is it recommended ? no, but you can't blame the wife for mistakenly pumping 87 in your GXP , and really, i know people feel good thinking the car was tuned for 87, while the engine can run 87, it operates the best with 93.

Let me repeat it again, the L76 was not designed to run 87,it doesn't matter how many labels GM slapped in the doors about it.

I posted exactly why 87 can be in the tank, in BOTH engines. You're assuming stuff based on labels, i'm saying how things happen in your PCM.

The L76 will knock on 87, just because you don't hear knock, doesn't mean isn't happening i cannot quantify how many horses would be lost because of the timing reduction though.

This is gearhead65 G8GT with 87 octane and stock timing tables. he only had a modified trans tune, look what happens when 87 is run through the L76 with the factory tune. same thing will happen to the LS3, and you won't hear the engine knocking, you may notice a sluggish engine if any, doubt most people would feel anything anyways though.

Can you run 87, yes, i wouldn't run 87 though, but both cars can run 87 regardless of the famous label, again, i wouldn't get caught with 87 unless the wife made the mistake. and trust me she won't make that mistake, she knows better now days , her 08 Honda Minivan runs on acetone for all i care, my G8 runs 91 , i just remind her everytime she drives my car once a month.
We talked about 87 octane back in 2010 in the HPT thread here: Well we need one..Hp tuners sharing thread..

No the car is fine, from my experiences in talking to other tuners, is that GM is running this engines super close to MBT, if not past MBT, to gain mpg, possibly a way to attain better emissions, and who knows what else do they get, with the increased temps in the combustion chamber as a result of the timing increase in the part throttle areas. They’ve also increased knock sensor sensitivity as a counter measure to fight the timing increase.

I have said this in the tuning section as well, GM is using this approach on all their LSx powered vehicles since 2005, and they’ve turned towards a more aggressive timing setup since 2007 or 2008. Their trucks line up exhibit crazy amounts of KR in logs with 87 as well, and even with 93, they still get knock while cruising, obviously they know something we don’t about this KR, because otherwise they wouldn’t offer a 100k miles warranty, and they’ve also increased mpg with the same engines over the years, starting with their 5.3L offer. for me, little amounts of KR is normal, but i don't like more than 2* at WOT.

I didn’t see crazy KR with the factory tune and 93 octane back when I was in Ohio (1-1.5* KR in cruising areas, and about 0.7-0.8* tops at WOT sometimes , so nothing to worry about that), car came in a truck to Southern Cali, with a full tank of gas, that gas lasted 2 days, LOL, pumped fresh 91 from Chevron, and within 5-10 min, KR went from almost nothing, to 4-6* in the areas where you see 10-15* in Gearhead65 logs, and at WOT, I had 4-5* KR as well in the 4000rpm area, as well as 5200rpm all the way to the shift point (I do runs in 3ed gear since 4th would be too much of a hassle).

Initially I thought the KR was fake, so why not dumb down the knock sensors a little to test. Bad idea, first time I ever heard an LSx engine rattling, so I knew the sensitivity of the sensors was not the problem, but the gas we have, combined with the factory timing, was simply too much.

I tried different gas stations and different gas brands as well to test, same results every time. Last test was to run 4 gallons of 100 octane gas in the tank, zero KR and factory timing. So, with that in mind, I simply adjusted the timing in the problem areas, to be what the engine needed for 91 octane without having those amounts of KR, keep in mind though, that even with less timing in the main timing tables, I have higher timing overall, than when I let the PCM remove timing based on the factory timing minus KR, if it makes sense.

Well, I just took a look at the spark tables for my LS3, and there is a high and low octane table. The timing is significantly lower on the low octane table. If the ECM calibration on the L76 is configured the same, then you are correct, increased performance would be had with high octane, and based on what I see in the LS3 calibration, it would be noticable.

As for the original question, 87 vs. 85, you would be running on the low octane table in either case.

Of course most modern vehicles, (if not all of them) would have 2 timing tables, Hi and Lo, it's needed for people who think is good to pump 85 octane at 7000ft elevatation and swear by internet stories of the air been less dense at elevation and having less air in the cylinder means you can run less octane. While this portion is true, this stories never talk about the fact that you can run more timing with less cyl airmass because the engine is not operating at full efficiency at higher elevations.

If the manufacturers left the timing static, like they did back in the 70's where timing was vaccum advanced 4-5* from startup to part throttle, and mechanically advanced at WOT, but no retarded based on bag gas, or 85 octane gas, then moving to Colorado and using 85 would probably work good depending on how much static timing was set in the distributor, most engines ran what, 18* timing when not using WOT ? now days, manufacturers can use an engine more efficiently increasing timing where the engine is less efficient, and will in fact increase the timing, the higher the elevation is, making this 85 octane theories useless.

If they wanted you to run 85 octane in Colorado, they'll do put a big sticker in red in the first page of the manual, and advertise how their 6.0 cars can run with ****ty 85 octane, Lol, do your engine a favor and run the highest octane you can, This cars are not Honda Civics, you paid for a 6.0L or 6.2L engine, the 6.0L runs 10:4 compression, and the 6.2 is bigger than the 6.0, why in the heck you want to run 87 octane in it ?Luckily for you, the factory timing/KR removal strategy allows you to run 85, "somewhat safely", and "somewhat optimally", at 7000ft, but not because the car was tuned for the lower octane gas.

Now, if you really wanted to take a look at some engine vitals, log Knock retard and knock learn factor, the last tells the pcm the % of the timing it should use from the Lo octane table, to make the engine operates at safer power levels with the most ****ty gas you can pump, to a point of course.

I guess I wonder if using regular unleaded on a non-performance car would land you in the "High octane" spark table with the "low octane" table being utilized when somebody has truly lousy gas. Based on the above posts, it looks like that's not the case for our cars.

I didn't realize that GM was recommending fuel that would put us into the low octane spark table. That being the case, I'm going to try a few tanks of the higher stuff, I think 89 might be the highest we can get here? Haven't paid much attention - wife drives the car, my only task has been to find diesel for the truck.

I have gone through many many tanks of premium to see ZERO difference between that and regular. As a matter of fact, the manual recommends regular fuel, which here in CO is 85 octane. So in a sense, the manual is recommending to run 85 octane (mid grade is 87). I have never seen 87 octane regular fuel here in CO. Is it an elevation thing? who knows.... My car runs just fine on 85 and when prices were Arm, Leg, and WTF, I ran the cheapest I could.

The owners manual states you can use 87 or higher... using lower grade fuels can cause spark knock if you hear it the manual says to put higher grade fuel in right away. Lower octane fuel that is lower than 87 can even make heavy knocking which means you'r engine needs service... Im p.o.'ed because i just got my car back from the dealer ship and put 85 octane in her. I always use 91 octane and will continue to do so.

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